Low coherence grazing incidence interferometry for profiling and tilt sensing

ABSTRACT

An optical system includes a photolithography system, a low coherence interferometer, and a detector. The photolithography system is configured to illuminate a portion of an object with a light pattern and has a reference surface. The low coherence interferometer has a reference optical path and a measurement optical path. Light that passes along the reference optical path reflects at least once from the reference surface and light that passes along the measurement optical path reflects at least once from the object. The detector is configured to detect a low coherence interference signal including light that has passed along the reference optical path and light that has passed along the measurement optical path. The low coherence interference signal is indicative of a spatial relationship between the reference surface and the object.

RELATED APPLICATIONS

The present application claims the benefit of U.S. provisionalapplication nos. 60/502,932, filed Sep. 15, 2003, for High SpeedScanning Interferometer for Surface Profiling and for Focus and TiltSensing, 60/502,933, filed Sep. 15, 2003, for Grazing IncidenceInterferometer for Profiling Surfaces Which May Have a Thin FilmCoating, 60/502,907, filed Sep. 15, 2003, for Triangulation Sensor forProfiling Surfaces Through a Thin Film Coating, 60/502,930, filed Sep.15, 2003, for Rapid Measurement of Surface Topographies in the Presenceof Thin Films, and 60/539,437, filed Jan. 26, 2004, for SurfaceProfiling Using An Interference Pattern Matching Template, each of whichapplications in incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to interferometric analysis of objects, such as tothe interferometric analysis of objects including a substrate having oneor more at least partially transparent layers.

BACKGROUND

Interferometric techniques are commonly used to measure the profile of asurface of an object. To do so, an interferometer combines a measurementwavefront reflected from the surface of interest with a referencewavefront reflected from a reference surface to produce aninterferogram. Fringes in the interferogram are indicative of spatialvariations between the surface of interest and the reference surface.

A scanning interferometer scans the optical path length difference (OPD)between the reference and measurement legs of the interferometer over arange of comparable to, or larger than, the coherence length of theinterfering wavefronts, to produce a scanning interferometry signal foreach camera pixel used to measure the interferogram. A limited coherencelength can be produced, for example, by using a white-light sourceand/or a spatially extended source. An exemplary technique is scanningwhite light interferometry (SWLI), which includes use of a broadbandsource. A typical scanning whit light interferometry (SWLI) signal is afew fringes localized near the zero optical path difference (OPD)position. The signal is typically characterized by a sinusoidal carriermodulation (the “fringes”) with bell-shaped fringe-contrast envelope.The conventional idea underlying SWLI metrology is to make use of thelocalization of the fringes to measure surface profiles.

Techniques for processing low-coherence interferometry data include twoprinciple trends. The first approach is to locate the peak or center ofthe envelope, assuming that this position corresponds to the zerooptical path difference (OPD) of a two-beam interferometer for which onebeam reflects from the object surface. The second approach is totransform the signal into the frequency domain and calculate the rate ofchange of phase with wavelength, assuming that an essentially linearslope is directly proportional to object position. See, for example,U.S. Pat. No. 5,398,113 to Peter de Groot. This latter approach isreferred to as Frequency Domain Analysis (FDA).

SUMMARY

Systems and methods described herein can be used to determine spatialproperties of objects having more than one interface.

In one aspect, the invention relates to an optical system comprising aphotolithography system configured to illuminate a portion of an objectwith a light pattern, the photolithography system comprising a referencesurface, a low coherence interferometer having a reference optical pathand a measurement optical path. Light that passes along the referenceoptical path reflecting at least once from the reference surface andlight that passes along the measurement optical path reflecting at leastonce from the object, and a detector configured to detect a lowcoherence interference signal comprising light that has passed along thereference optical path and light that has passed along the measurementoptical path. The low coherence interference signal is indicative of aspatial relationship between the reference surface and the object.

In some embodiments, the photolithography system includes anillumination optic having an illumination optic surface. Light of thelight pattern travels along an optical path that includes theillumination optic surface. The illumination optic surface and thereference surface are at least partially coextensive. The light thatpasses along the measurement optical path may reflect at least once fromthe portion of the object to be illuminated by the photolithographysystem.

The light of the low coherence interference signal that has passed alongthe reference optical path and the light of the low coherenceinterference signal that has passed along the measurement optical pathmay have a range of optical path length differences. The range may be atleast 20%, at least 50%, at least 75%, or more of a coherence length ofthe low coherence interferometer. The range may be at least as great asthe coherence length of the low coherence interferometer.

In some embodiments, the detector comprises a plurality of detectorelements each configured to detect a respective low coherenceinterference signal. Each low coherence interference signal may compriselight that has passed along a respective different portion of thereference optical path and light that has passed along a respectivedifferent portion of the measurement optical path. Each low coherenceinterference signal may be indicative of a spatial relationship betweena different point of the object and the reference surface.

The optical system may be configured to determine the spatialrelationship between each of the different points of the object and thereference surface based on at least a respective one of the lowcoherence interference signals. The optical system may include atranslation stage for manipulating a relative position and orientationbetween the object and the photolithography system. The processor may befurther configured to modify a relative position of the object and thephotolithography system based on the spatial relationships.

Another aspect of the invention relates to a method comprisingpositioning an object generally along an optical path of aphotolithography system, reflecting a first portion of light from alight source from a reference surface of the photolithography system,reflecting a second portion of light from the light source from theobject, and forming a low coherence interference signal comprising lightreflected from the reference surface and light reflected from theobject, the low coherence interference signal indicative of a spatialrelationship between the object and the imaging system.

In some embodiments, the method includes reflecting a respective firstportion of light from the light source from each of a plurality oflocations of the reference surface of the photolithography system,reflecting a respective second portion of light from the light sourcefrom each of a plurality of locations of the object, e.g., at a grazingangle of incidence, and forming plurality of low coherence interferencesignals. Each low coherence interference signal comprises lightreflected from a respective one of the different locations of thereference surface and light reflected from a respective one of thedifferent locations of the object. Each low coherence interferencesignal may be indicative of a spatial relationship between at least oneof the different locations of the object and the photolithographysystem.

In some embodiments, first and second portions of light can be reflectedafter positioning the object.

The method can further comprise changing a relative position of theobject and the reference surface based on the spatial relationship. Thereference surface can be a surface of an optical of the photolithographysystem. The photolithography system can be used to project anultraviolet light image onto the object. Light that forms theultraviolet image passes along an optical path including the surface ofthe optic.

The object can include a substrate and an overlying thin film having anouter surface and the forming can comprise combining light reflectedfrom the reference surface and light reflected from the outer surface ofthe thin film. The spatial relationship may be between the outer surfaceof the thin film and the photolithography system.

The light of the second portion of light from the light may besubstantially attenuated, e.g., absorbed, by the thin film.

The thin film can include photoresist, with the light of the secondportion of light from the light source having an energy insufficient toexpose the photoresist.

The object can include a substrate and a thin film having an outersurface. The with the forming and the forming comprises combining lightreflected from the reference surface and light reflected from thesubstrate, and the spatial relationship is between the substrate and theimaging system.

The object can be illuminated at Brewster's angle, which may enhancespatial information related to the substrate as opposed to the outersurface.

Another aspect of the invention relates to a system for determining aspatial property of an object. The system can include a light source, anoptical system configured to, illuminate the object at a grazing angleof incidence with a first portion of light from the light source, atleast some of the first portion of light reflecting from the object,combine, over a range of optical path differences, light reflected fromthe object and a second portion of light derived from the same lightsource, and a detector configured to detect the light combined over therange of optical path differences as a plurality of interference fringeseach having a peak amplitude, the range of optical path differencesbeing sufficient to modulate the peak amplitudes of the interferencefringes.

The range of optical path differences may be at least as great as acoherence length of the optical system.

Another aspect of the invention relates to a method includingilluminating an object a grazing angle of incidence with light from alight source. At least some of the illuminating light reflects from theobject. Light reflected from the object and a second portion of lightfrom the light source are combining over a range of optical pathdifferences. The light combined over a range of optical path differencesas a plurality of interference fringes each having a peak amplitude. Therange of optical path differences is sufficient to modulate the peakamplitudes of the interference fringes.

Another aspect of the invention relates to a including projecting afirst pattern of light on an object comprising a substrate and anoverlying thin film, imaging light of the first projected pattern thatis diffusely scattered by the substrate, and determining a spatialproperty of the object based on the diffusely scattered light.

The overlying thin film may be photoresist and determining a spatialproperty may include determining a position of a portion of the objectrelative to a photolithography system. The portion of the object may bean interface between the substrate and the overlying photoresist.

The first pattern of light may include comprises first and secondportions of light from the same light source and the first pattern oflight may be an interference pattern. The interference pattern caninclude a plurality of fringes modulated by an envelope and thedetermining a spatial property of the object comprises determining aposition of a portion of the envelope relative to the fringes.

The object can be repositioned based on the position of the portion ofthe envelope.

Some embodiments include projecting a reference pattern of light on areference surface, detecting light of the reference pattern projectedonto the reference surface, and the determining a spatial property ofthe object comprises determining a relative spatial property of theobject and the reference surface based on the detected light of thereference pattern. The object can be moved based on the relative spatialproperty.

Some embodiments further include modifying a property of the lightsource to project a second interference pattern comprising a pluralityof fringes having a substantially similar amplitude onto the object,imaging light of the second interference pattern that is diffuselyscattered by the substrate, and determining a second spatial property ofthe object based on the diffusely scattered light from the secondinterference pattern. The second spatial property may be a topography ofa portion of the object. The second spatial property may be indicativeof an absolute position of the object. At least the modifying can beperformed before projecting the first pattern of light.

Methods and systems described herein can be used to determine a spatialproperty of an object comprising a substrate comprising an overlyinglayer of photoresist having an outer surface. The spatial property canbe a of the outer surface. The methods and systems can change a relativeposition between a photolithography system and the object based on thespatial property.

Methods and systems described herein can be used to determine a spatialproperty of a portion of a liquid crystal display.

Methods and systems described herein can be used in scribing objects,e.g., by a laser. A spatial property of a scribed line formed on anobject by the scribing is determined. Further scribing of the object oranother object is performed. A parameter, e.g., a laser power, objectscan rate, or laser focus size is selected based on the spatial propertyof the scribed line.

Methods and systems described herein can be used to determine a spatialproperty of a structure formed during solder bump manufacturing. Thespatial property can be a spatial property of a portion of the objectnon-wettable by solder.

Another aspect of the invention relates to an apparatus including aphotolithography system configured to illuminate a portion of an objectwith an first light pattern. The photolithography system includes areference surface. The object includes a substrate and an overlying thinfilm. The apparatus also includes a positioner to change a relativeposition between the photolithography system and the object, a lightprojector configured to project a second light pattern on the overlyingthin film of the object, an optical system to image light of the secondlight pattern that is diffusely scattered by the substrate, and aprocessor configured to determine a spatial property of the object basedon the diffusely scattered light and change the relative positionbetween the photolithography system and the object.

Another aspect of the invention relates to an optical system including adetector comprising a plurality of elements arranged in at least twodimensions, and an optical system configured to illuminate a pluralityof spaced-apart points of an object with light from a light source, forma respective interference pattern corresponding to each illuminatedpoint, each interference pattern extending along a first dimension ofthe detector, the interference patterns for different points beingspaced apart along a second dimension of the detector.

Another aspect of the invention relates to an optical system including alight source, an array of detector elements extending in at least onedimension, an interferometer configured to illuminate, with a firstportion of light from the light source, a point of an object, and focuslight reflected from the illuminated point as an elongated focusextending along the first dimension of the array, and focus a secondportion of light form the source as a second focus extending along thefirst dimension of the array, the second focus and the elongated focusbeing at least partially coincident along the first dimension of thearray, an optical path difference (OPD) between the light reflected fromthe illuminated point and the second portion of light from the sourcevarying along the first dimension of the array by an amount greater thana coherence length of the light reflected from the illuminated point.

Another aspect of the invention relates to an interferometry method,comprising illuminating a plurality of spaced-apart points of an objectwith a first portion of light from a light source, at least some of thefirst portion of light reflecting from each of the spaced-apart points,and forming a plurality of interference patterns on a detector having aplurality of detector elements arranged in at least two dimensions,wherein each interference pattern comprises light reflected from arespective spaced-apart point of the object, each interference patternextends along a first dimension of the detector, and differentinterference patterns are spaced apart along a second dimension of thedetector.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs.

Other features, objects, and advantages of the invention will beapparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a cross-section of a measurement object including asubstrate and an overlying layer, e.g., a thin film.

FIG. 1 b is a top view of the object of FIG. 1 a.

FIG. 2 is a low coherence interference signal including first and secondinterference patterns with amplitudes that vary according to arespective envelope function.

FIG. 3 is a grazing incidence interferometry system.

FIG. 4 is a grazing incidence interferometry system.

FIG. 5 a illustrates shift between light passing along the measurementoptical path and reference optical path of the interferometry system ofFIG. 4 as a result of a tilt of a measurement object.

FIG. 5 b illustrates displacement vectors showing rotation between lightpassing along the measurement optical path and reference optical pathresulting from the measurement object tilt referred to in FIG. 5 a.

FIG. 6 is a grazing incidence interferometry system.

FIGS. 7 a and 7 b show detector images for a triangulation mode of agrazing incidence interferometry system.

FIG. 8 a is a grazing incidence interferometry system.

FIG. 8 b is a magnified view of a portion of the system of FIG. 8 a.

FIG. 9 a is a low coherence interferometry system.

FIG. 9 b is the interferometry system of FIG. 9 a viewed along theX-axis thereof.

FIG. 10 illustrates correspondence between illuminated points of ameasurement object and respective, elongated images detected for theilluminated points using the system of FIG. 9 a.

FIGS. 11 a and 11 b are exemplary components of a reference leg of theinterferometry system of FIG. 9 a.

FIGS. 12 a and 12 b are exemplary structures having copperinterconnects. FIGS. 12 a and 12 b showing the structure before andafter planarization, respectively.

FIGS. 13 a and 13 b are exemplary structures formed during solder bumpprocessing. FIG. 13 a showing the structure before addition of solder.FIG. 13 b showing the structure after addition of solder but prior toflowing the solder.

FIG. 14 is a portion of an exemplary liquid crystal display.

DETAILED DESCRIPTION

Embodiments of methods and systems described herein relate to the use ofinterferometry to measure a spatial property, e.g., a surfacetopography, a position, an orientation, and/or other characteristics, ofobjects having more than one interface, such as thin film structures,discrete structures of dissimilar materials, or discrete structures thatmay be underresolved by the optical resolution of an interferencemicroscope. Examples of interfaces include interfaces formed at theouter surface of an object or interfaces formed internally betweendissimilar materials. Spatial properties of objects having more than oneinterface are relevant to a variety of fields including flat paneldisplays, microelectronics, photolithography, thin filmscharacterization, and dissimilar materials analysis.

When an object with multiple interfaces is analyzed by interferometry,each interface can produce an interference pattern. If the interfacesare closely spaced, the interference patterns may overlap, distortingone another. The distortion can lead to erroneous determinations ofspatial properties of the object. As an example, consider efforts toposition a semiconductor wafer with overlying photoresist at a focusposition with respect to a photolithography system. The quality ofphotolithography is related to how precisely the wafer andphotolithography system can be positioned with respect to one another.However, the outer surface of the photoresist and the interface betweenthe photoresist and the wafer generate resulting interference patterns,which makes determining the exact position and orientation of thephotoresist outer surface or the wafer difficult. Consequently, thequality of photolithography may suffer.

Systems and methods described herein can determine a spatial property ofa selected interface of an object even in the presence of other adjacentor closely spaced interfaces. Some embodiments include illuminating theobject with light at a grazing angle of incidence α and detecting a lowcoherence interference signal including light reflected from the object.Angle α, taken with respect to a dimension extending normal to theobject, may be at least 60°, at least 70°, e.g., at least 80°.

Grazing incidence illumination can increase the reflectivity of theouter surface relative to other interfaces of the illuminated object.The reflectivity increase enhances interference patterns from the outerinterface as opposed to internal interfaces of the object. Hence,interference signals obtained with grazing incidence can be moresensitive than normal angle of incidence illumination to spatialproperties of the outer surfaces of objects. Embodiments for enhancinginterference patterns from the outer surface of a layer also (oralternatively) include selecting wavelengths of illuminating light thatare attenuated, e.g., absorbed, by the layer. Because the layer absorbsthe light, interference patterns from underlying interfaces arerelatively attenuated.

Also disclosed are embodiments in which grazing incidence techniques arecombined with low-coherence interferometry. As described further below,low-coherence interferometry data can be processed to provide spatialinformation about one or more interfaces of a complex sample, such as athin film structure. Such low coherence interferometry data can beobtained by using a spectrally broadband light and/or a spatiallyextended source. Accordingly, some embodiments for enhancing theinterference pattern from the outer surface of an object includeilluminating the object at grazing incidence with broadband light, e.g.,light having a full width half maximum (FWHM) of at least 6 nm, at least12.25 nm, at least 25 nm, at least 50 nm, at least 100 nm, or at least150 nm. When grazing incidence illumination is combined with broadbandlight, interference patterns obtained from the outer surface of anobject may be enhanced relative to interference patterns obtained frominternal interfaces of the object.

Enhancing the interference patterns resulting from an outer surface of alayer can benefit a number of applications, e.g., the photolithographypositioning mentioned above. For example, grazing incidence methods andsystems described herein can determine the thickness of a photoresistlayer overlying a substrate with a relative accuracy of about 1% orbetter for thin films, e.g., films about 400 nm thick or thicker. In aparticular example, a thickness of a 450 nm thick layer of XF1 157 nm UVphotoresist overlying a silicon film having a thickness of 450 nm wasdetermined with an error of ±4.9 nm using light having a nominalwavelength of 600 nm, a FWHM of 200 nm, an angle of incidence α of 80°and a Δα of +/−3°.

Embodiments of grazing incidence illumination are not limited toenhancing interference patterns resulting from an outer surface of anobject. Interference patterns from internal interfaces can also beenhanced. For example, an object can be illuminated at Brewster's anglewith light polarized in a plane defined by the angle of incidence. Inthis case, Brewster's angle is determined by the optical properties,e.g., refractive index, of the overlying layer, and by the wavelengthsof the illuminating light. At Brewster's angle, interference patternsfrom underlying interfaces are enhanced relative to interferencepatterns from the outer surface.

Grazing angle of incidence illumination provides other advantagescompared to normal angle of incidence illumination. For example, anillumination beam having a grazing angle of incidence can be used toilluminate an object in close proximity with other objects or systems.For example, grazing incidence interferometers described herein canintroduce an illumination beam between an the imaging optics of aphotolithography system and an object with photoresist to be imaged.Hence, benefits of grazing incidence with respect to thin film analysiscan be realized in situ for complex, crowded working environments. Allof the interferometers and optical systems discussed herein may be usedto determine a spatial property of an object, e.g., a subset of theobject to be imaged, with respect to a photolithography apparatus and,with feedback, modify the relative position and/or orientation of theobject and photolithography apparatus. Additionally, each interferometerand optical system may include a reference surface that is itself asurface of a photolithography apparatus, e.g., a surface of an optic ofthe photolithography apparatus.

Accordingly, grazing incidence illumination allows enhancinginterference patterns from a selected interface (whether an outersurface or an internal interface) to increase the accuracy of spatialproperties determined based on the interference patterns. Methods andsystems for determining one or more spatial properties of objects aredescribed below. We begin with a general description of an object havingmore than one interface and describe interference patterns that might beobtained from such an object using, e.g., a low coherence grazingincidence interferometer. Then, embodiments of optical systems arediscussed.

Referring to FIG. 1 a, an object 30 includes a substrate 32 and a layer34. Object 30 includes a plurality of interfaces as occur betweenmaterials of different refractive index. For example, anobject-surroundings interface 38 is defined where an outer surface 39 oflayer 34 contacts the environment surrounding object 30, e.g., liquid,air, other gas, or vacuum. A substrate-layer interface 36 is definedbetween a surface 35 of substrate 32 and a bottom surface 37 of layer34. Surface 35 of the substrate may include a plurality of patternedfeatures 29. Some of these features have the same height as adjacentportions of the substrate but a different refractive index. Otherfeatures may extend upward or downward relative to adjacent portions ofthe substrate. Accordingly, interface 36 (and, of course, interface 38)may exhibit a complex, varying topography.

Referring to FIG. 2, an interference signal 90 is exemplary of a lowcoherence interference signal that may be obtained from objects havingclosely spaced interfaces using systems and methods described herein.Interference signal 90 includes first and second overlappinginterference patterns 92,96 respectively resulting from outer surface 39and interface 36. The X-axis of interference signal 90 corresponds to anoptical path difference (OPD) between light reflected from the objectand reference light. An interferometer can vary the OPD by scanning,e.g., by moving an optic and/or the object to vary the optical pathtraveled by the light reflecting from the object or the reference light.An interferometer may, alternatively or in combination, vary the OPD bydetecting a spatial distribution of light reflected from the object andthe reference light with the OPD varying as a function of spatialposition.

Interference patterns 92,96 are modulated as a function of OPD byrespective coherence envelopes 97,95, which have similar shapes andwidths. In the absence of the low coherence envelope, the fringes 98,99typically have similar amplitudes. The width of an interference patternenvelope corresponds generally to the coherence length of the detectedlight. Among the factors that determine the coherence length aretemporal coherence phenomena related to, e.g., the spectral bandwidth ofthe source, and spatial coherence phenomena related to, e.g., the rangeof angle of incidence of light illuminating the object.

Typically, the coherence length decreases as: (a) the spectral bandwidthof the source increases and/or (b) the range of angles of incidenceincreases. Depending upon the configuration of an interferometer used toacquire the data, one or the other of these coherence phenomena maydominate or they may both contribute substantially to the overallcoherence length. In some embodiments, grazing angle of incidenceinterferometers described herein illuminate objects with broadband lighthaving a range Δα of angles of incidence. The light source may beextended. The range Δα can be ±20° or less, ±10° or less, ±5° or less,or ±3° or less. In some embodiments, the illumination numerical apertureis 0.2 or less, 0.1 or less, 0.07 or less, 0.06 or less, e.g., 0.05 orless. Because of the grazing angle of incidence, spatial coherencephenomena contribute to the observed interference signals at lowerranges of angles of incidence than that for normal incidence. This isbecause the spatial coherence phenomena are related to changes in pathlength of oblique rays, which scale inversely with the cosine of theangle of incidence α. Description of interference signals includingcontributions from spatial coherence phenomena are described in U.S.patent application Ser. No. 10/659,060, titled Interferometry Method forEllipsometry, Reflectometry, and Scatterometry Measurements, IncludingCharacterization of Thin Films, which is incorporated herein byreference. In some embodiments, both spatial and temporal coherencephenomena contribute to the coherence length, which may desirablyattenuate interference patterns resulting from internal interfaces.

The coherence length of an interferometer can be determined by obtainingan interference signal from an object having a single reflectingsurface, e.g., not a thin film structure. The coherence lengthcorresponds to the full width half maximum of the envelope modulatingthe observed interference pattern. As can be seen from FIG. 2,interference signal 90 results from detecting light having a range ofoptical path differences that varies by more than the width of thecoherence envelopes and, therefore, by more than the coherence length ofthe detected light. In general, a low coherence interference signalincludes interference fringes that are amplitude modulated by thecoherence envelope of the detected light. For example, the interferencepattern may be obtained over an OPD for which the amplitude of theobserved interference fringes differs by at least 20%, at least 30% orat least 50% relative to one another. For example, fringe 98 has a peakamplitude that is about 50% less than a peak amplitude of a fringe 99.In some embodiments, the low coherence interference signal is detectedover a range of OPD's that is comparable to or greater than thecoherence length. For example, the range of OPD's may be at least 2times greater at least 3 times greater than the coherence length. Insome embodiments, the coherence length of the detected light is on theorder of the height variations of features of the object, e.g., on theorder of couple of microns or less.

Interference signals obtained using methods and systems described hereincan be processed in a number of ways to determine a spatial property ofthe object. In some embodiments, processing the interference signalincludes transformation of the signal to an inverse dimension. Suchtransformation can include Fourier transformation of a signal. Thetransformation may be performed during Frequency Domain Analysis (FDA)or an extension thereof. Exemplary FDA methods are described in U.S.Pat. No. 5,398,113 entitled “METHOD AND APPARATUS FOR SURFACE TOPOGRAPHYMEASUREMENTS BY SPATIAL-FREQUENCY ANALYSIS OF INTERFEROGRAMS” and U.S.patent application Ser. No. 10/795,808 filed Mar. 8, 2003 and entitled“PROFILING COMPLEX SURFACE STRUCTURES USING HEIGHT SCANNINGINTERFEROMETRY,” the contents said patent and patent application beingincorporated herein by reference. It should be understood, however, thatprocessing an interference signals does not require transformation. Forexample, the maximum of an interference envelope can provide spatialproperty information even without transformation of the interferencesignal.

As seen in FIG. 2, a portion 94 of interference signal 90 is dominatedby contributions from interference pattern 92 as opposed to interferencepattern 96. As discussed above, interference pattern 92 results from theouter surface 39 of object 30. A spatial property, e.g., a positionand/or height, of surface 39 can be determined based upon portion 94,which constitutes only a subset of the entire interference signal 90.Methods and systems for locating and analyzing such subsets ofinterference signals and described in U.S. patent application Ser. No.09/008,721, titled METHODS AND SYSTEMS FOR INTERFEROMETRIC ANALYSIS OFSURFACES AND RELATED APPLICATIONS by Peter de Groot, filed Sep. 15,2004. This application is incorporated in its entirety herein byreference.

Interferometry systems and methods for obtaining and processinginterference signals, e.g., in some embodiments, low coherenceinterference signals, from objects, such as objects having a pluralityof interfaces are discussed next.

Referring to FIG. 3, an optical system 100 illuminates uses diffractiveoptics to illuminate an object at grazing angle of incidence. System 100includes an interferometry system 101 and an illumination system 150,which systems are configured for use with measurement object 30 andother objects, such as objects lacking any overlying layer or objectsincluding a plurality of such layers. While not a low coherenceinterferometer, system 101 can determine a characteristic, e.g., aspatial or optical property, of measurement object 30 using grazingincidence illumination and is illustrative of the benefits of such aconfiguration. With reference back to FIGS. 1 a and 1 b, the spatial oroptical property may be related to surface 39 of layer 34 or a subset40, thereof. Alternatively, or in combination, the spatial or opticalproperty may be related to interface 36, e.g., to surface 35 ofsubstrate 32. System 101 typically employs phase-shifting techniques toprovide information related to object 30.

Illumination system 150 is typically configured to illuminate surfaceobject 30 with light 167, such as to image a selected pattern, e.g., acircuit pattern onto surface 38. Returning to FIG. 3, various aspects ofsystems 101 and 150 are discussed below.

Interferometry system 101 is a grazing incidence system arranged foranalyzing a measurement object. A light source 11, which may be a lamp,a light-emitting diode, a multimode laser diode or a gas laser generatesa beam 102. After passing through expansion optics 103, the beam 102produces an initial illumination wavefront 104. A diffractive-optic beamsplitter 105, which may be a linear phase grating with zero-ordersuppression, separates initial illumination wavefront 104 into areference wavefront 115 and a measurement wavefront 110. The twowavefronts 115 and 110 result from opposite grating orders, e.g., thepositive and negative first grating orders, and consequently travel indivergent directions. Reference wavefront 115 reflects once fromreference surface 130 prior to traveling to a diffractive-optic beamcombiner 170, which may be similar to diffractive-optic beam splitter105.

As discussed further below, reference surface 130 may also be configuredas a portion of illumination system 150, such as a portion of projectionoptics that direct patterned light onto object 30, e.g., a subset 40_(i) of the object. In any event, reference surface 130 may be opticallyflat, e.g., to about 1/15 of the average wavelength of wavefront 115 orhave a known surface shape. For example, the projection objects may havean arcuate surface with known curvature.

Referring also to FIG. 1 a, a measurement ray 181 illustrates theinteraction of light of measurement wavefront 110 with object 30. In thepresence of layer 34 and substrate 32, a portion of measurement ray 181impinges and reflects from layer surface 38 at a grazing angle α, andthen travels as a reflected ray 181′. As seen in FIG. 3, a reflectedmeasurement wavefront 110′ including ray 181′ propagates todiffractive-optic beam combiner 170 where it recombines with thereference wavefront 115 to form an output wavefront 120. Turning back toFIG. 1 a, a second portion of measurement ray 181 may penetrate layer 34and reflect from interface 36 at the surface of substrate 32 as areflected ray 181″. The light reflected from interface 36 generates asecond reflected wavefront (not shown) that propagates generally alongthe same path as wavefront 181′ but spaced apart by an amount Δs along adimension perpendicular to the propagation path. The second wavefrontwould also combine and interfere with reference wavefront 115.

In some embodiments, system 100 is configured to obtain a singlemeasurement wavefront reflected from surface 39 at the interface 38between layer 34 and the environment around object 30 and to attenuateor exclude measurement wavefronts reflect from interface 36 betweensubstrate 32 and layer 34. Measurement wavefronts reflected fromsubstrate-surface layer interface 36 can be attenuated or excluded byselecting a source 111 wavelength that is substantially absorbed by thesurface layer 34. Because the source light is absorbed by the surfacelayer, there is essentially no reflected wavefront arising from thesubstrate-surface layer interface 38. Instead, substantially the onlyreflected wavefront arises from the surface layer-surroundings interface38, e.g., an interface between the surface layer and air, other gas, orvacuum surrounding object 30. Hence, ray 181″ and any associatedwavefronts would be attenuated or excluded.

In some embodiments, the surface layer 34 is photoresist configured tobe exposed by ultraviolet light emitted by illumination system 150.Typically, a component of the photoresist layer, e.g., a solvent or anoptically active component of the resist itself, absorbs lower energyradiation, such as visible, near-infrared, or infrared radiation,without exposing the optically active component. Such non-exposingabsorptions can result from vibrational excitation of the resist ratherthan electronic excitation as by ultraviolet light. In any event, theresist absorbs a portion of the measurement light and system 101generates a measurement wavefront including measurement ray 181′ andresulting substantially only from the interface 38 between the surfacelayer 34 and the surroundings. Such a wavefront carries informationabout spatial properties of the interface 38, e.g., a topography and orposition of surface 39. The spatial properties may be given withreference to a portion of illumination system 150, such as surface 130of projection optics thereof.

In some embodiments, system 100 is configured to obtain a singlemeasurement wavefront reflected from surface 36 at the interface betweensubstrate 32 and layer 34 and to attenuate or exclude measurementwavefronts reflected from interface 38 at the surface of layer 34 andthe surroundings. Measurement wavefronts reflected from interface 38 canbe attenuated or excluded by selecting the polarization P and angle ofincidence α of the incident light, e.g., measurement ray 181, tominimize the intensity of measurement ray 181′ relative to measurementray 181″ reflected from surface 36. For example, the polarization P canbe configured to be parallel to the plane of incidence at surface 38.Alternatively or in combination, the angle of incidence α can beconfigured to be equal to Brewster's angle, the angle at whichreflection from surface 39 is minimized for the aforementionedpolarization. In any event, the reflection from surface 39 is reduced oreliminated and system 101 generates a measurement wavefront includingmeasurement ray 181″ and resulting substantially only from the interface36 between the substrate 32 and surface layer 34. Such a wavefrontcarries information about spatial properties of the interface 36, e.g.,a topography and or position of surface 36 of the substrate. The spatialproperties may be given with reference to a portion of illuminationsystem 150, such as surface 130 of projection optics thereof.

Output wavefront 120, whether including information about surface 36,surface 39 or both surfaces, travels through a lens 171 and an imaginglens 173 to a viewing screen, e.g., a two-dimensional detector such as aCCD 175 where an image 190 of sample surface 160 forms. Detector 175 isinclined obliquely to achieve proper focus across the detected image.The tilt also reduces foreshortening in image 190 caused by imagingsample surface 160 at grazing angle α. Image 190 contains interferencefringe information related to the topography of sample surface 160. Insuch interpretations the equivalent wavelength Λ relevant to thesefringes is given by Λ=λ/cos(α), where λ is the nominal wavelength ofwavefront 104.

The term “diffractive-optic”, as used herein, is intended to includediffraction gratings, binary optics, surface-relief diffractive lenses,holographic optical elements, and computer-generated holograms. Thesedevices can function in transmission or in reflection as beam splittersand combiners. They may suppress unwanted diffraction orders, e.g., thezero'th order transmission, so as to reduce scattered light and improveefficiency. Methods of fabricating diffractive-optical devices includediamond machining, coherent beam interference (holography), injectionmolding, and advanced micro-lithographic techniques. Diffractive-opticsare recognized by those skilled in the art as distinct from refractiveand reflective optical elements such as lenses, prisms, mirrors andplate beam splitters.

When measurement wavefront 110′ and reference wavefront 115 cometogether to form output wavefront 120, overlapping portions ofmeasurement wavefront 110′ and reference wavefront 115 originate fromsubstantially the same portion of initial illumination wavefront 104.For example, a reference ray 185 and measurement ray 181 recombining ata point 189 on diffractive-optic beam combiner 170 originatesubstantially from the same point 109 on diffractive-optic beam splitter105. As a consequence, aberrations or spatial incoherence in initialillumination wavefront 104 generate, at most, weak effects on theinterference fringes observed in image 190. As a further consequence,small defects in beam expansion optics 103 or distortions resulting fromair turbulence produce, at most, weak effects on the analysis of image190. As a further consequence, deviation from flatness in thediffractive-optic beam splitter 105 or the diffractive-optic beamcombiner 170 generate, at most, weak effects on the interference fringesobserved in image 190.

Any additional characteristic of system 101 is that equivalentwavelength Λ is substantially independent of the wavelength λ of source111. This may be understood as follows. From the geometry of the system101 and well-known properties of diffraction gratings, it can be shownthat the angle of incidence α is given bycos(α)=λ/D,where D is the grating pitch, i.e., the linear separation betweengrating lines, of the diffractive-optic beam splitter 105 and thediffractive-optic beam combiner 170. Accordingly, the equivalentwavelength Λ=D. Thus, different source wavelengths λ produce the sameequivalent wavelength Λ, which is equal to the grating pitch.

The optical path traversed by measurement wavefront 110 is substantiallyequal to the optical path traversed by reference wavefront 115. Thisfacilitates the use of multimode laser diodes or other devices having awavelength range sufficient to reduce the effects of spurious fringepatterns and speckle noise characteristic of single-mode lasers or otherhigh-coherence spectrally narrow-band or monochromatic devices, e.g.,wavelength range <1 nm, when used as sources for interferometry. Thesubstantial equality of the optical paths traversed by the wavefronts115 and 110 also desensitizes this first embodiment to instability inthe wavelength of source 101, which might otherwise be a problem forlaser diodes, which can oscillate between lasing modes unexpectedly.

One method for obtaining interferometry data from system 101 is aphase-shifting method for which a number of interference states aremeasured by the detector 175. Phase-shifted data can be obtained byintroducing an OPD between the measurement and reference beams. Such anOPD can be generated by in-plane translation of one of the diffractiongratings, such as in a direction perpendicular to the grating lines.

Another method for introducing an OPD includes introducing a slightdifference in grating period between the two gratings. This creates atilt between the two wavefronts that interfere at the detector,resulting in a spatial carrier fringe pattern. In this case one of anumber of techniques can be used to extract height information from asingle exposure of the detector. For example, a Fourier transform phasemeasurement can be used.

Turning to illumination system 150, an illumination source 152 emitslight 160, which is received by a beam conditioner 154. The beamconditioner directs a conditioned beam 162 to illumination optics 156,which transmit light 164 through a mask or reticle 158 onto interface 38of object 30 via projection optics 157.

Illumination source 152 is typically an ultraviolet source, e.g., alaser emitting an ultraviolet laser beam. In some embodiments, source152 emits light 160 having a wavelength including 248 nanometers (nm),193 nm, or 157 nm. Source 152 may be a pulsed laser or a continuous wavelaser. Beam conditioner 154 conditions light 160 received fromillumination source 152, such as to produce a collimated beam 162 havinga defined cross-section. Exemplary beam conditions can include, e.g.,refractive and/or reflective optics, such as described in U.S. Pat. No.5,631,721, titled Hybrid Illumination System for Use inPhotolithography, by S. Stanton, et al., incorporated herein byreference in its entirety.

Illumination optics 156 receive conditioned light 162 and output light164 as an illumination field, which irradiates mask or reticle 158.Optics 156 can be configured to provide an illumination field having auniform irradiance and to preserve the angular distribution andcharacteristics of the illumination field at the reticle as the size ofthe illumination field is varied. Mask or reticle 159 typically includesa pattern, e.g., a circuit pattern, to be projected onto an object. Forexample, the optics 156 can focus the pattern onto a subset 40 _(i) ofthe object.

Still referring to FIG. 3, the interferometry system 101 provides alarge working distance to the interface 38 and reference surface 130.Working distance refers to the distance between a sample surface and theclosest optical component. The large working distance in grazingincidence system 101 means that sample interface 38 can be convenientlypositioned without concern for possible damage to sample interface 38 orother components.

System 100, like all interferometry systems discussed herein, mayinclude a positioning stage 119 configured to position object 30 withrespect to another object, e.g., with respect to surface 130 ofprojection optics of illumination system 150. Stage 119, which is undercomputer control with feedback from data acquired by interferometrysystem 101, provides translational and rotational positioning to bringobject 30 into a desired spatial relationship with the other object. Forexample, based on interferometry data acquired from wavefronts reflectedfrom surface 39 or a subset 40 _(i) thereof, system 100 can maneuverobject 30 so that surface 39 or a subset 40 _(i) thereof is brought intoa desired spatial relationship with respect to surface 130, e.g.,parallel thereto and/or at a certain distance therefrom. System 100 canmaneuver object 30 into a similar spatial relationship between surface36 and surface 130.

Referring to FIG. 4, a low coherence interferometry system 250 isanother example of a grazing incidence interferometer than can be usedto determine a spatial property of an object having a plurality ofinterfaces. In this embodiment, the system includes a low coherenceinterferometer with optics can be compactly arranged so that the systemoccupies a small footprint and can be used in conjunction with othersystems used to manipulate an object.

System 250 determines a spatial property of an object 252 byilluminating the object with light at a grazing angle of incidence.System 254 includes a light source 254, which can be a broadband and/orextended source emitting a light beam 255. Exemplary sources include awhite light LED having a central wavelength of 550 nm and a full widthhalf maximum (FWHM) of 120 nm and a xenon arc lamp having a 200 nm FWHM.In general, a ratio of a FWHM of light beam 255 to a central wavelengthof beam 255 is at least 5%, at least 10%, at least 15%, e.g., at least20%.

Beam 255 is received by an optic, e.g., a lens 256, which prepares acollimated beam 257. The focal length of lens 265 is (in the embodimentshown) 150 mm, which corresponds to an illumination numerical apertureof 0.004. A beam splitter 258 splits beam 257 into a measurement beam259 and a reference beam 261. Measurement beam 259 reflects from amirror 262 and impinges upon object 252 at grazing angle α of incidence.For example, object 252 may be a wafer bearing photoresist to beilluminated by a photolithography system. System 250 has a 25×80 mmfield of view of the surface of object 252.

Light 263 reflected by object 252 is received by a beam splitter 264.The optical path of the measurement beam can include a compensator 260,which can be used to modify the distance through dense media, e.g.,glass, traveled by the measurement beam and/or to modify a lateraldisplacement of beam 259. With reference to FIG. 4, beams 259 and 263occupy a plane parallel to the X-Y plane.

Beam splitter 264 combines light 263 and reference beam 261 to form abeam 265, which is detected by a detector, e.g., an imaging detector 276having a plurality of detector elements, e.g., pixels. Imaging optics,e.g., telecentric optics 272,274 image the beam 265 at the detector 275so that different pixels detect light corresponding to different pointsof object 252. Reference beam 261 can follow a path including mirrors276,270 and a compensator 268, which can serve the same function ascompensator 260.

System 250 can be configured so that an optical path traveled by beam259 between beam splitters 258 and 264 is identical to an optical pathtraveled by reference beam 162 between beam splitters 258 and 264.Hence, an optical path difference (OPD) between beams 259,261 may benegligible or zero. Interference is observed when the OPD between thebeams is less than a coherence length of the detected light. Moreover,system 250 can be configured to detect beams 259,261 over a range ofoptical path differences. For example, system 250 can be configured sothat one of beams 259,261 travels an initially longer optical path.System 250 then moves at least one component to scan the OPD, e.g.,until the OPD reaches zero or until the other beam travels a longeroptical path. In some embodiments, system 252 detects beams 259,261 overan OPD range that varies by an amount at least as great as a coherencelength of light detected by detector 276. Different detector elements ofdetector 276 record an interference signal as a function of OPD. Theinterference signals may resemble interference signal 90 by includingone or more interference patterns modulated by an envelope. Theinterference signals can be analyzed as discussed with respect to system50 to determine a spatial property of an object, e.g., a spatialproperty of an outer surface of a layer of photoresist.

Because source 254 is an extended source, it is possible to misalignsystem 250 so that the OPD varies as a function of position across thereference and measurement beams 259,261, which form images of the sourceat a pupil plane 276′ of optics 272,274. In such misalignment, theoptical axes of the reference and measurement legs may be parallel butnot coextensive, e.g., parallel but laterally displaced. This results inlateral shear of the illumination bundle. Shear in the XY plane of about50 μm can reduce fringe contrast by 50%. To reduce or prevent such lostof fringe contrast, system 250 can be aligned to correct lateral shearto within 10 μm or less, 5 μm or less, e.g., 2 μm or less.

Another source of contrast loss can result from rotation of beam 263about its axis for propagation without a corresponding rotation of beam261 about its axis of propagation. The rotation of beam 263 can resultfrom rotation of the object about an axis U extending through theobject. In fact, alignment seeking to reduce lateral shear can introducesuch rotation. The axis U occupies the plane defined by beams 259 and263. An axis V is normal to the plane defined by beams 259 and 263.Tilting object 252 about the U or V axis laterally displaces the imageof the source at pupil 276′. Tilting the about about the U axis,however, also rotates beams 263 about its axis.

Referring to FIGS. 5 a and 5 b, a simulation of relative displacement of(a) an image of source 254 formed by light passing along the referenceleg ∘ and an image of the source 254 formed by light passing along themeasurement leg ⋄ of a grazing incidence interferometer 250 isdiscussed. The source images are as formed at pupil 276′ of telecentricoptics 272,274 by beams 261,263. The simulation is for light incidentupon an object at an 80° angle of incidence with a numerical aperture of0.004. In FIG. 5 a, the images for the reference and measurement legsshow distortion resulting from an object rotation of 0.5° about theU-axis. The measurement leg ⋄ image is laterally shifted by 0.44 mm withrespect to the reference leg image ∘. In FIG. 5 b, the rigid body motionof the measurement leg image is subtracted to obtain displacementvectors that demonstrate that beam 263 is, in addition to the lateraldisplacement, rotated by 0.96° relative to beam 261. The maximumhorizontal and vertical distortion of the image reflected from theobject is about ±21 μm, which is about ±5% of the 0.44 mm shift. Therotation introduces a ±5% variation in the pitch and orientation ofinterference fringes that would be detected. Hence, the rotationresulting from object tilt about the U-axis would create a substantialcontrast variation over the field of view of the detector. System 250can be aligned for minimal lateral and rotational shear by, e.g.,confining the optical axes of each segment of the reference andmeasurement legs of the system within the same plane. Object defocusdoes not affect fringe contrast (to a first approximation) for lownumerical aperture interferometers.

Referring to FIG. 6, a low coherence interferometry system 300determines a spatial property of an object by illuminating the object ata grazing angle of incidence. System 200 includes a low coherenceinterferometer having a measurement leg and a reference leg. Asdiscussed below, a pair of optical flats act as a beam splitter and beamcombiner. Light traveling along the reference leg travels within a gapbetween the optical flats. Light traveling along the measurement leg canexit the gap between the optical flats to reflect from the object beforereentering the gap.

System 200 includes a source 202, which may be broadband and/orextended. Source 202 generates a beam 204. An optic 206 receives beam204 and outputs a collimated beam 207, which impinges upon a pair ofspaced apart optical flats 208,210. Beam 207 is refracted by flat 208and passes through a gap 233 and impinges upon flat 210 at site 235′.Gap 233 has a different refractive index than either of flats 208,210.In some embodiments, gap 233 includes a gas, e.g., air, a liquid, e.g.,water, or a vacuum. The flats 208,210 can be formed of any suitableoptical medium, such as glass or fused silica.

A portion 220 of beam 207 is reflected from site 235′ by flat 210. Aportion 222 of beam 207 is refracted at site 235′ by flat 210 andimpinges upon object 30 at a grazing angle of incidence α. Beam 222 isreflected by object 30 and impinges upon and is refracted by flat 210.For example, object 30 may be a wafer bearing photoresist to beilluminated by a photolithography system. Beam 222 reflected from object30 and beam 220 combined at site 235″ of flat 210 to form combined beam224 within gap 233. The combined beam impinges upon and is refracted byflat 208. Imaging optics 226,228 image the combined beams on a detector230, which can be a two-dimensional imaging detector including aplurality of pixels. Each pixel of detector 230 detects light reflectedfrom a different point of object 30. Hence, different pixels can detectinterference signals sensitive to spatial properties of differentportions of object 30.

System 200 includes a reference leg and a measurement leg. Beam 220travels a reference optical path between sites 235′ and 235″. Beam 22travels a measurement optical path between sites 235′ and 235″. An OPDdifference between the reference and measurement optical paths can bevaried in several ways. In some embodiments, system 200 includespiezoelectric spacers 237 configured to vary a thickness of gap 233. Asspacers 237 vary gap 233, detector 230 detects images including aplurality of points of object 30. The OPD can also be scanned by movingobject 30 with respect to flats 208,210.

In some embodiments, an OPD is achieved by introducing a wedge into atleast one of flats 208,210, e.g., flat 208. The upper and lower surfacesof a flat with such a wedge are not parallel. The wedge tilts thewavefront of the reference beam 220 with respect to the measurement beam222. Accordingly, the combined beam 224 forms a spatial interferencepattern at detector 230. The spatial interference pattern can beprocessed to determine a height of surface 38 over a line of pointsparallel to the X-axis. Thus, in a single detector acquisition, system200 can provide spatial information about object 30. Additionally,object 30 can be positioned absolutely with respect to system 200 bymonitoring a lateral location of peak interference contrast of thespatial interference pattern.

Whether the OPD is varied by scanning or to form a spatial interferencepattern, the combined beam 224 may be detected over an OPD range that isat least as great as a coherence length of the detected light.Accordingly, system 200 can detect interference patterns modulated by anenvelope indicative of the coherence length of the light as discussedwith respect to interference signal 90. Interference patterns obtainedwith system 200 can be analyzed as discussed elsewhere herein todetermine a spatial property of an object.

Optical flats 208,210 have optical and mechanical properties that allowsystem 200 to image a desired field view, e.g., 100 mm×100 mm, of object30. In some embodiments, the flats are formed of fused silica and are atleast 2 mm, at least 5 mm, e.g., at least 10 mm thick.

Surfaces of optical flats 208,210 can be modified to determine theamount of light that is reflected or refracted by each flat. Forexample, portions 230 of optical flats 208,210 can be modified with ananti-reflection coating configured to increase the relative amount ofrefracted light compared to the amount of reflected light. The coatingcan be a broadband coating matched to the emission spectrum of source202. Light incidence upon portions 235 of optical flat 235 is bothreflected and refracted. Accordingly, portions 235 may be uncoated orcan be coated to obtain a desired ratio of reflection and refraction. Aportion 231 of optical flat 208 can have a highly reflective, e.g.,metal or dielectric, coating.

Grazing incidence interferometers discussed herein can be used todetermine an absolute position of an object with respect to theinterferometer. The reference and measurement legs of theinterferometers operate as triangulation sensors. To operate a grazingincidence interferometer as a triangulation sensor, an aperture, e.g., aslit, is positioned in the illumination optics such that the aperture isimaged onto the object surface Interferometer 101 (FIG. 3) is shown withsuch an aperture 96 although any interferometer discussed herein may beso modified.

The aperture acts as the field stop of a microscope. With reference toFIG. 7 a, first and second images 200 a,201 a of the aperture are imagedonto the detector of the interferometer. Image 200 a is an object imagecorresponding to light reflected from the object. Image 201 a is areference image corresponding to light passing along the reference pathof the interferometer. The aperture is small enough that the images 200a, 201 a occupy only a fraction of the field of view of the detector.The object is positioned with respect to the interferometer by a stagingmechanism, e.g., a translation stage 119 of interferometer 101, that canbe accurately displaced along its normal.

When images 200 a, 201 a are first recorded, the object is typically notpositioned so that the OPD of the measurement and reference legs iszero. Accordingly, the images 200 a,201 a are spaced apart from oneanother as seen in FIG. 7 a. The relative positions of the object andthe interferometer are modified, as by displacing the object along itsnormal. The object displacement is known precisely and accurately basedmovement of the translation stage. A second pair of images 200 b (asecond object image),201 b (a second reference image) are then obtained.

The detector signal including images 200 a,201 a (FIG. 7 b) and thedetector signal including images 200 b,201 b (FIG. 7 b) are processed tocorrelate the position of object images 200 a,200 b in the two detectorsignals. For example, relative positions of object images images 200a,200 b can be determined in terms of spatial units at the detector (forexample a number of pixels). The object image displacement as a functionof object displacement can be determined from the relative positions ofimages 200 a,200 b. Once the relationship between the displacement ofthe object image and object displacement is determined, the displacementbetween object image 200 b and reference image 201 b is determined. Theobject can then be translated to the position of zero OPD (at which theobject image and reference image overlap) based on the displacementbetween images 200 b,201 b and the relationship between object imagedisplacement and object displacement.

As a result of these steps, the object can be positioned with respect tothe interferometer with an absolute position of the object surface knownto better than a fraction of a fringe of an interference pattern. Thegrazing incidence interferometer can be switched back to interferometrymode by removing the slit. The position of the object surface can thenbe refined using interference signals as described herein. Thetriangulation based on the object and reference images is insensitive toobject tilt since the object surface is imaged onto the detector.

Referring to FIGS. 8 a and 8 b, and optical system 400 is configured toimage light diffusely scattered from an interface between a substrateand a layer overlying the substrate. By imaging the diffusely scatteredlight, the optical system is sensitive to spatial properties of theinterface rather than the outer surface of the object. System 400 can beused to position a substrate having a thin film relative to aphotolithography system.

Optical system 400 includes a structured light projector 408, whichprojects a pattern 402 of light onto the object, and a telecentricimaging system 406, which images the pattern 402 onto a detector 420.Imaging system 406 detects light from the object arising at an anglethat avoids light 407 specularly reflected from interface 36 or 38. Forexample, system 406 may detect light 409 diffusely scattered generallyalong an optical axis aligned at an angle α with respect to light 407.

The diffuse scattering may arise from patterned features 29 of substrate32. Such features can be small with respect to a wavelength of light,e.g, 1 μm or less, or 0.5 microns or less. The top surface 39 of layer34, however, tends to be smooth. Accordingly, the diffuse scattering canbe localized with respect to the substrate-layer interface 36. Theobject top surface 39 and each individual interface, e.g., interface 36,inside the object reflect the illumination light in a specular directionat the angle of incidence, away from the entrance pupil of the imagingsystem. Hence, the diffusely scattered light 409 is the dominatecomponent of the detected light and system 350 can provide spatialinformation indicative of the substrate-layer interface based on thedetected diffusely scattered light.

In the embodiment shown, structured light projector 404 is a Michelsoninterferometer including a source 408, which emits a light beam 411,which is collimated by an optic 410, e.g., a lens positioned at itsfocal length from source 408. In some embodiments, source 408 isbroadband and/or spatially extended, such as a light emitting diode.Source 408 may be narrowband or, as discussed below, switchable betweennarrow and broadband. A beam splitter 415 splits light beam 411 intofirst and second portions, which respectively reflect from mirrors 412and 414. Beam splitter 415 recombines at least some light of the firstand second portions to form a combined beam 417.

A telecentric optical relay 416 relays the combined beam 417 to theobject at a grazing angle of incidence α. The first and second portionsof the combined beam 417 impinge on the object with an OPD that differsacross the object (FIG. 8 b). Hence, the first and second portions ofthe combined beam interfere at the wafer surface 36 forming interferencefringes, i.e., pattern 402. Interference fringes can be observed even ifsource 408 is broadband and/or spatially extended because the first andsecond portions of the combined beam 417 can have identical path lengthsbetween beam splitter 415 and the object. Because the source can exhibitsome level of incoherence, pattern 402 and the detected image avoidcoherent artifacts (speckles). Although structured light projector 404forms a pattern based on interference fringes, the projector may form apattern by projecting an image without relying upon interference to formfeatures of the pattern.

Returning to FIGS. 7 a and 7 b, the fringes formed by patterned lightprojector extend parallel to the x-axis and are spaced apart along they-axis of the object. Diffusely scattered light 409 from the fringes isimaged on detector 420, which may be a two-dimensional detector such asa CCD. Several approaches may be used to determine a spatial property ofthe object based on the detected fringes.

In a spatial carrier approach, the detector is typically configured todetect a plurality of parallel fringes, e.g., the fringes 402 areprojected across the entire field of view of the detector. Heightvariations (such as steps or surface discontinuities of the substrate)will shift the phase of the fringes. With reference to FIG. 8 b, aspacing Δs between the fringes is a function of the wavelength of thelight of beam 417 and the angle of incidence α. Even if the object andthe beam 417 are fixed relative to one another, the angle of incidence αwill vary as a function of the surface topography. In particular, thefringe spacing Δs will decrease or increase for portions of interface 36that are respectively tilted toward or away from beam 417 so as todecrease or increase the angle α. Hence, spatial properties of theobject can be determined from even a single image of the pattern 402.

In the spatial-carrier embodiments, one or more detector images ofpattern 402 are obtained. A spatial property of the detected portions ofthe surface, e.g., the height of one or more points of the substrate, isdetermined based upon the fringes 402, e.g., based upon spacing Δs. Thedetected image can be analyzed using, e.g., FDA via transformation ofthe image or directly from the image itself. For example, the spacing Δscan be determined directly from the detected fringes and related to thetopography of the object.

In a phase shifting approach, multiple detector exposures of the patternare recorded while the pattern 402 is shifted across the object. Thepattern can be shifted by, e.g., modifying an optical path length of oneof the first and second portions of the combined beam. For example, oneof the mirrors 412,414 can be translated or tilted using a piezoelectrictransducer. The phase of light detected from each of many points of thesubstrate is indicative of the topography of the substrate.

Exemplary approaches suitable for determining a spatial property ofinterface 36 based on the projected pattern 402 are described in“Interferogram analysis: digital fringe pattern measurement techniques,”D. Robinson, G. Reid Eds., IOP Publishing, 1993, the contents of whichare incorporated herein by reference.

In some embodiments, device 350 is operated with a source that isbroadband and/or spatially extended. In this case, a temporal andspatial coherence envelope modulates the amplitude of the fringes ofpattern 402. The envelope modulates the fringes typically even withinthe field of view of detector 406. Accordingly, detector 420 images aplurality of parallel fringes, each extending parallel to the x-axis andeach having a different intensity than the adjacent fringes. The maximumof the envelope corresponds with the location of zero OPD between thefirst and second portions of the combined beam. The position of themaximum of the envelope relative to the detected fringes is indicativeof the absolute position and orientation of the object. Accordingly, themaximum of the envelope can be used to absolutely localize the objectwith respect to system 350.

In some embodiments system 400 includes a reference surface 427, whichcan be located adjacent object 30 and at approximately the same heightas surface 35 or surface 39. The object 30 and reference surface 427 maybe fixed with a transition stage 119. The reference surface may beconfigured to diffusely scatter light. For example, the referencesurface may be an etched or frosted glass surface. In use with thereference surface, system 350 is typically operated with a broad bandsource to provide a modulating envelope. The reference surface 427 ispositioned so that the pattern is projected onto the reference surfaceand imaged by the detector. The stage 119 (and thus reference surface427) are positioned so that the envelope maximum has a predeterminedrelationship with the observed fringe pattern, e.g., centrally located.Then, the object 30 is translated into the field of view of thedetector. A change in the position of the envelope maximum relative tothe remainder of the fringes is indicative of a height differencebetween the reference and object 30. The stage can be moved so that theenvelope maximum is once again in the predetermined relationship withthe observed fringe pattern. Thus, the object can be positionedabsolutely with respect to a reference surface.

In some embodiments, system 400 can be switched between a spectrallybroadband and narrowband source. The broadband source operates asdiscussed above creating an amplitude modulated interference patternthat varies even within the field of view of the detector. The object ispositioned based upon the location of the maximum of the interferencepattern so that the position of zero OPD coincides with a predeterminedportion of the object. Hence, the object can be absolutely positionedwith respect to system 400. Once the object is so positioned, the system400 switches to a narrowband source having a coherence lengthsufficiently long that the fringes are essentially unmodulated withinthe field of view of the detector. The properties of the fringes areanalyzed to determine a spatial property of the object. The system canuse a spectral filter to switch the beam 411 between narrowband andbroadband light.

Referring to FIGS. 9 a and 9 b, an interferometry system 50 can obtainlow coherence interference signals from object 30 and other objects,such as objects lacking any overlying layer or objects including aplurality of such layers. System 50 illuminates a plurality of points ofan object with an illumination stripe extending in a first dimension anddetects an interference pattern resulting from each point. Theinterference patterns extend along a first dimension of a detector andare spaced apart along a second dimension of the detector. System 50can, therefore, obtain interference patterns without moving any elementthat modifies an optical path length difference. Although notnecessarily operated in grazing incidence mode, system 50 rapidly obtaininterference signals from a plurality of object points for an objectbeing subjected to other processing steps.

A light source, e.g., a light source 52 emits a beam 54 of lightelongated in a direction parallel to the X-axis. Source 52 may be abroadband source having a full width at half maximum (FWHM) bandwidththat is at least 5%, at least 10%, at least 15%, or at least 20% of anominal wavelength of the source. In some embodiments, source 52 has anominal wavelength of between about 300 nm and about 1000 nm, e.g.,between about 500 and about 600 nm.

Beam 54 is received by a cylindrical lens L1, which has a majorlongitudinal axis parallel both to the X-axis and beam 54 and a minorlongitudinal axis parallel to the Y-axis. Lens L1 collimates beam 54 inthe Y-Z plane and transmits a collimated beam 55 to a beam splitter B1,which reflects a first portion of the collimated beam 55 to acylindrical lens L2 and transmits a second portion of the collimatedbeam 55 to a cylindrical lens L3. Beam splitter B1 is typicallynon-polarizing. Lens L2 has a major longitudinal axis parallel to theX-axis and a minor longitudinal axis parallel to the Z-axis. Lens L3 hasa major longitudinal axis parallel to the X-axis and a minorlongitudinal axis parallel to the Y-axis.

With reference to FIG. 9 a, lens L2 focuses the reflected portion of thecollimated beam received from beam splitter B1 onto the object 30 to anelongated object focus, e.g., an illumination line 67, parallel to theX-axis. A ratio of a dimension of illumination line 67 taken along amajor axis thereof (e.g., parallel to the X-axis) to a dimensionperpendicular to the major axis thereof (e.g., parallel to the Y-axis)can be at least 5, at least 10, at least 25, at least 50, e.g., at least100. Such dimensions of illumination line 67 may be determined fromlocations corresponding to 25% of the maximum illumination intensity.Illumination line 67 may have a generally uniform or slowly varyingintensity along its length and irradiates a plurality of points ofobject 30. The points may be considered as being spaced apart along anillumination dimension of the object 30, e.g., the X-axis.

With reference to FIG. 9 b, light reflected by object 30, e.g., lightreflected from interface 36 and/or surface 39, is received by lens L2,which collimates the reflected light in the Y-Z plane and transmits thecollimated light to beam splitter B1, which transits a portion of thelight to a beam splitter B2, which is typically non-polarizing. Lightthat passes from beam splitter B1, reflects from object 30, and passesto beam splitter B2 travels a measurement optical path.

The second portion of the collimated beam transmitted by B1 is receivedby lens L3, which transmits a converging beam to a 90° roof mirror 62.The converging beam comes to a focus f1, which is typically elongatedand extends parallel to the X-axis. The roof mirror 62 transmits adiverging beam to a cylindrical lens L4, which has a major longitudinalaxis aligned with the X-axis and a minor longitudinal axis aligned withthe Y-axis. Lens L4 transmits a beam collimated in the Y-Z plane to beamsplitter B2. Lenses L2, L3, and L4 may have identical opticalproperties, e.g., focal lengths. Lenses and other optics of system 50may be achromatic. Light passing from beam splitter B1 and to beamsplitter B2 via roof mirror 62 travels a reference optical path. Themeasurement optical path defines a measurement leg of an interferometer89 of system 50. The reference optical path defines a reference leg ofthe interferometer 89 of system 50.

Beam splitter B2 combines light from the measurement and reference legsof interferometer 89 and transmits a combined beam 59. The contributionsto combined beam 59 from both the measurement and reference legs arecollimated in the Y-Z plane. Beam 59 may be diverging in the X-Z plane.A cylindrical lens L5 receives the combined beam 59 and focuses thelight on a detector, which is typically a two dimensional detector 71including a plurality of pixels 73, arranged in rows extending along theY-axis and columns extending along the X-axis. Different columns have adifferent Y-coordinate. Different rows have a different X-coordinate.Detector 71 may be a charge coupled device (CCD) or other imagingdetector. Lens L5 has a major longitudinal axis aligned with the Y-axisand a minor longitudinal axis aligned with the Z-axis. Accordingly, lensL5 has substantially more focusing power in the X-Z plane than in theY-Z plane, e.g., lens L5 may have essentially no focusing power in theY-Z plane.

With reference to FIG. 10, interferometer 50 images light from themeasurement and reference legs onto detector 71. Light reflected fromeach illuminated point of object 30 is imaged as an elongated focus,e.g., a detection line. For example, light from an illuminated point 81a is imaged to a corresponding elongated focus 81 b along a pixel row 83of detector 71 and light from an illuminated point 85 a is imaged to acorresponding elongate focus 85 b along a pixel row 87 of detector 71.Light from focus f1 of the reference leg is imaged as a reference focus99, which overlaps elongated foci corresponding to the illuminatedpoints of object 30.

A ratio of a dimension of each elongated focus 81 b,85 b taken along amajor axis thereof (e.g., parallel to the Y-axis) to a dimensionperpendicular to the major axis thereof (e.g., parallel to the X-axis)can be at least 5, at least 10, at least 25, at least 50, e.g., at least100. As seen in FIG. 4, elongated foci corresponding to points spacedapart along the illumination dimension of object 30 are spaced apartalong a first detection dimension of detector 71, e.g., elongated foci81 b,85 b corresponding to spaced apart points 81 a,85 a are imaged intodifferent rows of pixels 73 and are spaced apart along the X-axis ofdetector 71. The major axis of each elongated focus extends along asecond detection dimension generally perpendicular to the firstdetention dimension, e.g., elongated foci 81 b,85 b extend across aplurality of columns of pixels 73 and extend along the Y-axis ofdetector 71. Accordingly, interferometer 50 can image light reflectedfrom a plurality of points extending along an illumination dimension ofan object as a two-dimensional image. Illuminated points spaced apartalong the illumination dimension are imaged as elongated foci spacedapart along a first dimension of the image and extending along thesecond dimension of the image.

Returning to FIG. 9 a, the reference and measurement legs ofinterferometer 89 may be configured to have a nominally equal pathlength when lenses L1-L5 are centered with respect to the optical pathsof interferometer 89. In the equal path length state, an OPD betweenelongated foci 81 b,85 b and reference focus 99 may be constant alongthe rows of detector 71 (FIG. 4). An optical medium having a refractiveindex greater than 1, e.g., a silica or glass plate 97, can bepositioned along the reference optical path between beam splitters B1,B2and the lenses L3, L4 in order to match the optical path in such mediaof the two interferometer legs.

In some embodiments, an OPD between the measurement and reference legsof interferometer 98 is introduced by translating lens L4 by an amountΔd perpendicular to the reference optical path, e.g., by translatinglens L4 parallel to the Z-axis (FIG. 10). The translation of an optic,e.g., lens L4, perpendicular to the reference optical path causes thebeam passing along the reference leg to deviate, e.g., tilt, by an angleθ with respect to the beam passing along the reference leg in theabsence of such a translation, e.g., the reference beam may deviate withrespect to a true reference optical path. The deviation of the angle θmay be contained in a single plane, e.g., the X-Y plane.

The angular deviation of the reference beam creates an OPD variationbetween each elongated focus 81 b,85 b and reference focus 99. Inparticular, the OPD between the measurement and reference optical pathsvaries along the major axis of each elongated focus on the detector,e.g., along foci 81 b,85 b. For example, an OPD between the measurementand reference optical paths for light imaged to a column 91 of detector71 is different from an OPD for light imaged to a column 93 (FIG. 10).Hence, in the embodiment shown, the OPD between the measurement andreference optical paths varies as a function of the Y-coordinate ofdetector 71. In some embodiments, the OPD is a linear function of the Ycoordinate, e.g., the OPD is a linear function of the column of detector71 at which the light is imaged. For example, with reference to FIG. 10,the OPD varies, e.g., linearly, along the rows of detector 71 and,therefore, along the major dimension of the elongated foci 81 b,85 b.Because the major axis of the elongated focus corresponding to eachilluminated points of object 30 extends generally across the rows ofdetector 71, each row of pixels records an interference signal, whichincludes one or more interference patterns. Typically, each interferencepattern of an interference signal results from a particular interface ofthe object. The detector pixel in each column along a given rowcorresponds to a different OPD. Hence, one detector dimensioncorresponds to a line of object positions, while the other dimensionprovides multiple phase-shifted samples of the interference patterngenerated by each object point. Scanning the object in the Y-directionallows sequentially profiling the entire object surface.

Returning to FIG. 2, interference signal 90 is exemplary of theintensity variation of light detected by pixels along rows of detector71, e.g., along detector row 83 for point 81 a of object 30. The OPDdifference of light detected along a row can vary by an amount at leastas large as the coherence length of the detected light. For example, asshown, the range of optical path differences of light detected along thepixels of one or more rows can be larger than the full width of theenvelope modulating each of one or more detected interference patterns.Because of the low-coherence nature of the interference signal, aposition and/or height of point 81 a can be established withoutambiguity with respect to the system 50.

Because interference signals obtained with system 50 are spread outspatially for different object points, a single exposure of the detectorallows profiling an entire line of points at the object surface. Onceobject 30 has been brought into an initial focus with respect toillumination line 67, an interference pattern for each of a plurality ofpoints spaced apart along a first dimension of object 30 can be acquiredwithout moving any portion of system 50. Accordingly, in someembodiments, interferometer 89 does not have moving parts or does notmove any parts during the acquisition of interference patterns from eachof a plurality of spaced apart object points and can be manufactured asa rigid or fixed assembly.

The range of illumination angles at the object 30 can be madearbitrarily small by increasing the focal length of the optics ofinterferometer 89. Accordingly, variations in the optical properties ofthe object surface that are related to angle of incidence areeffectively reduce or eliminated. Each detected interference pattern isequivalent to the signal that would be detected at a single detectorpixel if the optical path in the test or reference leg were scannedcontinuously.

As discussed above, the OPD between the measurement and reference beamsmay be obtained by decentering an optic, e.g., lens L4 with respect toan optical path of the reference beam. In some embodiments, obtaining anoptical path difference includes adjusting roof mirror 62 to introducean angular deviation to the reference beam. In some embodiments,obtaining an OPD includes rotating or tilting beam splitter B2 about theX-axis.

Referring to FIGS. 9 a and 9 b, system 50 is, in some embodiments,configured to reduce or eliminate a wavefront inversion of the referencebeam with respect to the measurement beam. Wavefront inversion can bereduced or eliminated by having the same number of reflections in eachleg of the interferometer or an even multiple thereof. As seen in FIG.11 a, wavefront inversion introduced by roof prism 62 is evidenced bythe inversion of beams 77 a,77 b upon reflection. As seen in FIG. 11 b,a three mirror reflector 62 b does not introduce wavefront inversion asevidenced by the paths of beams 77 c,77 d. Reducing wavefront inversioncan enhance accuracy of interferometer 89.

Referring back to FIG. 9 a, source 52 may include a slit 57 having amajor dimension extending generally parallel to the X-axis. Theprojected width of the slit 57 in the Y-direction defines the lateralresolution of system 50 in the Y-direction while the numerical apertureof lens L5 in the X-dimension defines lateral resolution in theX-direction. Slit 57 can be defined by, for example, mechanical apertureor a linear fiber array. In some embodiments, source 52 includes aspatial filter to limit the divergence of the illumination beam in theXZ plane. An exemplary spatial filter includes a slit 61 having a majordimension extending generally parallel to the Z-axis and telecentriclenses 80 a,80 b, which image a Lambertian emitter 70 onto slit 57.

Although illumination line 67 typically has a uniform intensity, in someembodiments, the line may be non-uniform. For example, light source 52may include the ends of a plurality of optical fibers arranged in anelongated array, e.g., slit. Lenses L1,L2 may image light emitted fromthe elongated array to illuminate object 30 with light having anon-uniform intensity.

PROCESSORS

Any of the computer analysis methods described above can be implementedin hardware or software, or a combination of both. The methods can beimplemented in computer programs using standard programming techniquesfollowing the method and figures described herein. Program code isapplied to input data to perform the functions described herein andgenerate output information. The output information is applied to one ormore output devices such as a display monitor. Each program may beimplemented in a high level procedural or object oriented programminglanguage to communicate with a computer system. However, the programscan be implemented in assembly or machine language, if desired. In anycase, the language can be a compiled or interpreted language. Moreover,the program can run on dedicated integrated circuits preprogrammed forthat purpose.

Each such computer program is preferably stored on a storage medium ordevice (e.g., ROM or magnetic diskette) readable by a general or specialpurpose programmable computer, for configuring and operating thecomputer when the storage media or device is read by the computer toperform the procedures described herein. The computer program can alsoreside in cache or main memory during program execution. The analysismethod can also be implemented as a computer-readable storage medium,configured with a computer program, where the storage medium soconfigured causes a computer to operate in a specific and predefinedmanner to perform the functions described herein.

EXEMPLARY APPLICATIONS

The low coherence interferometry methods and systems described above mayused for any of the following surface analysis problems: simple thinfilms; multilayer thin films; sharp edges and surface features thatdiffract or otherwise generate complex interference effects; unresolvedsurface roughness; unresolved surface features, for example, asub-wavelength width groove on an otherwise smooth surface; dissimilarmaterials; polarization-dependent properties of the surface; anddeflections, vibrations or motions of the surface or deformable surfacefeatures that result in incident-angle dependent perturbations of theinterference phenomenon. For the case of thin film, the variableparameter of interest may be the film thickness, the refractive index ofthe film, the refractive index of the substrate, or some combinationthereof. For the case of dissimilar materials, for example, the surfacemay comprise a combination of thin film and a solid metal, and a fit ofthe angle-dependent surface properties would be made to a library oftheoretical predictions which would include both surface structure typesto automatically identify the film or the solid metal by a match to thecorresponding interference intensity signal. Exemplary applicationsincluding objects and devices exhibit such features are discussed next.

PHOTOLITHOGRAPHY

In many microelectronics applications, photolithography is used topattern a layer of photoresist overlying a substrate, e.g., a siliconwafer. In terms of object 30, substrate 32 may correspond to a wafer andlayer 34 with a thin layer of photoresist. The interface 38 correspondswith the upper surface of the photoresist and interface 36 correspondswith the wafer-photoresist interface. Surface 35 of the substrate mayhave a plurality of patterned features of varying topography and/orcomposition that underlie the photoresist. Accordingly, the object mayexhibit a plurality of interfaces underlying the photoresist outersurface.

A photolithography apparatus images a pattern onto the object. Forexample, the pattern may correspond with elements of an electroniccircuit (or the negative of the circuit). After imaging, portions of thephotoresist are removed revealing the substrate underlying the removedphotoresist. The revealed substrate can be etched, covered withdeposited material, or otherwise modified. Remaining photoresistprotects other portions of the substrate from such modification.

To increase manufacturing efficiencies, more than one device issometimes prepared from a single wafer. The devices may be the same ordifferent. Each device requires that a subset of the wafer be imagedwith a pattern. In some cases, the pattern is sequentially imaged ontodifferent subsets. Sequential imaging can be performed for severalreasons. Optical aberrations can prevent achieving adequate patternfocus quality over larger areas of the wafer. Even in the absence ofoptical aberrations, the spatial properties of the wafer and photoresistmay also prevent achieving adequate pattern focus over large areas ofthe wafer. Aspects of the relationship between the spatial properties ofthe wafer/resist and focus quality are discussed next.

Referring to back to FIG. 1 b, object 30 is shown with a number Nsubsets 40 _(i), each smaller than a total area 41 the object to beimaged. Within each subset 40, spatial property variations, e.g., heightand slope variations of the wafer or photoresist, are typically smallerthan when taken over the total area 41. Nonetheless, the wafer orphotoresist of different subsets 40 _(i) typically have differentheights and slopes. For example, layer 34 exhibits thicknesses Δt₁ andΔt₂, which vary the height and slope of surface 39 (FIG. 1 a). Thus,each subset of the object may have a different spatial relationship withthe photolithography imager. The quality of focus is related to thespatial relationship, e.g., the distance between the object and thephotolithography imager. Bringing different subsets of the object intoproper focus may require relative repositioning of the object andimager. Because of the object height and slope variations, proper subsetfocus cannot be achieved solely by determining the position andorientation of the object with respect to a portion of the object thatis remote to the imaged subset, e.g., a side 43 of the object.

Proper focus can be achieved by determining a spatial property of anobject within a subset of the object to be imaged (or otherwiseprocessed). Once the position of the subset has been determined, theobject (and/or a portion of the photolithography imager) can be moved,e.g., translated, rotated, and/or tilted, to modify the position of thesubset with respect to a reference, e.g., a portion of thephotolithography imager. The determination and movement (if necessary)can be repeated for each subset to be imaged.

The determination of the spatial property of the subset can includedetermining a position and/or height of one or more points of an outersurface of a thin layer of the object, the one or more points lyingwithin the subset of the object to be imaged. For example, the positionand orientation of the outer surface 39 of subset 40 ₂ (FIG. 1 a) can bedetermined based upon the positions of points 42 ₁-42 ₃ within thesubset. The determination of the spatial property of the subset to beimaged can include using an interferometer to illuminate the subset withlight and detecting an interference signal including light reflectedfrom the illuminated subset. In some embodiments, a plurality of subsetsare simultaneously imaged with light to obtain a plurality ofinterference signals. Each interference signal is indicative of one ormore spatial properties of a subset. Thus, the interference signals canbe used to prepare an image indicative of the topography of the objectover a plurality of the subsets. During photolithography of the subsets,the wafer is positioned based upon the topography of the individualsubsets as determined from the plurality of interference signals. Hence,each subset can be positioned for optimum focus with respect to thephotolithography apparatus.

Detecting an interference signal from each subset of an object to beimaged can include detecting light reflected from the subset andreference light over an OPD range that is at least as large as acoherence length of the detected light. For example, the light may bedetected at least over its coherence length. In some embodiments, theinterferometer is configured so that the light reflected from theilluminated subset is dominated by light reflected from either an outerinterface (such as outer surface 39) or an inner interface (such asinterface 36). In some embodiments, a spatial property of an object isdetermined based on only a portion of the interference signal. Forexample, if the interference signal includes two or more overlappinginterference patterns, a spatial property of the object can bedetermined based upon a portion of one of the interference patterns thatis dominated by contributions from a single interface of the object.

COPPER INTERCONNECT STRUCTURES AND CHEMICAL MECHANICAL POLISHING

It is becoming common among chip makers to use the so-called ‘dualdamascene copper’ process to fabricate electrical interconnects betweendifferent parts of a chip. This is an example of a process which may beeffectively characterized using a suitable surface topography system.The dual damascene process may be considered to have six parts: (1) aninterlayer dielectric (ILD) deposition, in which a layer of dielectricmaterial (such as a polymer, or glass) is deposited onto the surface ofa wafer (containing a plurality of individual chips); (2) chemicalmechanical polishing (CMP), in which the dielectric layer is polished soas to create a smooth surface, suitable for precision opticallithography, (3) a combination of lithographic patterning and reactiveion etching steps, in which a complex network is created comprisingnarrow trenches running parallel to the wafer surface and small viasrunning from the bottom of the trenches to a lower (previously define)electrically conductive layer, (4) a combination of metal depositionsteps which result in the deposition of copper trenches and vias, (5) adielectric deposition step in which a dielectric is applied over thecopper trenches and vias, and (6) a final CMP step in which the excesscopper is removed, leaving a network of copper filled trenches (andpossible vias) surrounded by dielectric material.

Referring to FIG. 12 a, a device 500 is exemplary of the a filmstructure resulting from the deposition of a dielectric 504 over copperfeatures 502 deposited on a substrate 501. The dielectric 504 has anon-uniform outer surface 506 exhibiting height variations therealong.Interference signals obtained from device 500 can include interferencepatterns resulting from surface 506, an interface 508 between copperfeatures 502 and dielectric 504, and an interface 510 between substrate501 and dielectric 504. The device 500 may include a plurality of otherfeatures that also generate interference patterns.

Referring to FIG. 12 b, a device 500′ illustrates the state of device500 after the final CMP step. The upper surface 506 has been planarizedto a surface 506′, and interface 508 may now be exposed to thesurroundings. Interface 510 at the substrate surface remains intact.Device performance and uniformity depends critically on monitoring theplanarization of surface 504. It is important to appreciate that thepolishing rate, and therefore the remaining copper (and dielectric)thickness after polishing, depends strongly and in a complex manner onthe polishing conditions (such as the pad pressure and polishing slurrycomposition), as well as on the local detailed arrangement (i.e.,orientation, proximity and shape) of copper and surrounding dielectricregions. Hence, portions of surface 506 over copper elements 502 mayetch at different rates that other portions of surface 506.Additionally, once interface 508 of copper elements 502 is exposed, thedielectric and copper elements may exhibit different etch rates.

The ‘position dependent polishing’ is known to give rise to variablesurface topography on many lateral length scales. For example, it maymean that chips located closer to the edge of a wafer on aggregate arepolished more rapidly than those located close to the center, creatingcopper regions which are thinner than desired near the edges, andthicker than desired at the center. This is an example of a ‘waferscale’ process nonuniformity—i.e., one occurring on length scalecomparable to the wafer diameter. It is also known that regions whichhave a high density of copper trenches polish at a higher rate thannearby regions with low copper line densities. This leads to aphenomenon known as ‘CMP induced erosion’ in the high copper densityregions. This is an example of a ‘chip scale’ processnon-uniformity—i.e., one occurring on a length scale comparable to (andsometimes much less than) the linear dimensions of a single chip.Another type of chip scale nonuniformity, known as ‘dishing’, occurswithin single copper filled trench regions (which tend to polish at ahigher rate than the surrounding dielectric material). For trenchesgreater than a few microns in width dishing may become severe with theresult that affected lines later exhibit excessive electricalresistance, leading to a chip failure.

CMP induced wafer and chip scale process nonuniformities are inherentlydifficult to predict, and they are subject to change over time asconditions within the CMP processing system evolve. To effectivelymonitor, and suitably adjust the process conditions for the purpose ofensuring that any nonuniformities remain within acceptable limits, it isimportant for process engineers to make frequent non-contact surfacetopography measurements on chips at a large number and wide variety oflocations. This is possible using embodiments of the interferometrymethods and systems described above.

In some embodiments one or more spatial properties, e.g., the topographyof surface 506 and/or the thickness of dielectric 504, are monitored byobtaining low coherence interference signals from the structure beforeand/or during CMP. Based on the spatial properties, the polishingconditions can be changed to achieve the desired planar surface 506′.For example, the pad pressure, pad pressure distribution, polishingagent characteristics, solvent composition and flow, and otherconditions can be determined based on the spatial properties. After someperiod of polishing, the spatial property can again be determined andthe polishing conditions changed as needed. The topography and/orthickness is also indicative of the end-point at which, e.g., surface504′ is achieved. Thus, the low coherence interference signals can beused to avoid depressions caused by over polishing different regions ofthe object. The low coherence interference methods and systems areadvantageous in this respect because spatial properties of the device,e.g., the relative heights of the surface of the dielectric (a) overcopper elements 502 and (b) over substrate surface 510 but adjacentcopper elements 502 can be determined even in the presence of themultiple interfaces.

SOLDER BUMP PROCESSING

Referring to FIGS. 13 a and 13 b, a structure 550 is exemplary of astructure produced during solder bump processing. Structure 550 includesa substrate 551, regions 502 non-wettable by solder, and a region 503wettable by solder. Regions 502 have an outer surface 507. Region 503has an outer surface 509. Accordingly, an interface 505 is formedbetween regions 502 and substrate 501.

During processing a mass of solder 504 is positioned in contact withwettable region 503. Upon flowing the solder, the solder forms a securecontact with the wettable region 503. Adjacent non-wettable regions 502act like a dam preventing the flowed solder from undesirable migrationabout the structure. It is desirable to know spatial properties of thestructure including the relative heights of surfaces 507,509 and thedimensions of solder 504 relative to surface 502. As can be determinedfrom other discussions herein, structure 550 includes a plurality ofinterfaces that may each result in an interference pattern. Overlapbetween the interference patterns prevents accurate determinate of thespatial properties using known interference techniques. Application ofthe systems and methods discussed herein allow the spatial properties tobe determined.

Spatial properties determined from structure 550 can be used to changemanufacturing conditions, such as deposition times for layers 502,503and the amount of solder 504 used per area of region 503. Additionally,heating conditions used to flow the solder can also be changed based onthe spatial properties to achieve adequate flow and or prevent migrationof the solder.

LIQUID CRYSTAL DISPLAYS

Referring to FIG. 14, a passive matrix LCD 450 is composed of severallayers. The main parts are two glass plates 452,453 connected by seals454. A polarizer 456 is applied to the front glass plate 453 in order topolarize incoming light in a single direction. The polarized lightpasses through the front glass plate 453. An Indium Tin Oxide (ITO)layer 458 is used as an electrode. A passivation layer 460, sometimescalled hard coat layer, based on SiOx is coated over the ITO 458 toelectrically insulate the surface. Polyimide 462 is printed over thepassivation layer 460 to align the liquid crystal fluid 464. The liquidcrystal fluid is sensitive to electric fields and changes orientationwhen an electric field is applied. The liquid crystal is also opticallyactive and rotates the polarization direction of the incoming light. Thecell gap Δg, i.e., thickness of the liquid crystal layer 464, isdetermined by spacers 466, which keep the two glass plates 452,453 at afixed distance. When there is no electric potential from the front plate453 to the rear plate 452, the polarized light is rotated 90° as itpasses through the liquid crystal layer 464. When an electric potentialis applied from one plate to the other plate the light is not rotated.After the light has passed through the liquid crystal layer 464, itpasses through another polyimide layer 468, another hard coat layer 470,a rear ITO electrode 472, and the rear glass plate 452. Upon reaching arear polarizer 474, the light either transmitted through or absorbed,depending on whether or not it has been rotated 90°. The cell 450 mayinclude filters 476 or other colorizing elements to provide a colordisplay.

The cell gap Δg determines to a great extent the optoelectricalproperties of the LCD, e.g., the contrast ratio and brightness. Cell gapcontrol during manufacturing is critical to obtaining uniform, qualitydisplays. The actual cell gap may differ from the dimensions of spacers466 because, during assembly, pressure or vacuum is applied to introducethe liquid crystal medium, seals 454 cure and may change dimensions, andthe added liquid crystal medium generates capillary forces betweenplates 452,453. Both before and after adding the liquid crystal medium464, surfaces 480,482 of plates 452,453 reflect light that results in aninterference pattern indicative of the cell gap Δg. The low coherencenature of the interference signal either itself or in combination withthe described interference signal processing techniques can be used tomonitor properties of the cell including the cell gap Δg duringmanufacture even in the presence of interfaces formed by other layers ofthe cell.

An exemplary method can include obtaining a low coherence interferencesignal including interference patterns indicative of the cell gap Δgprior to adding layer 464. The cell gap (or other spatial property ofthe cell) is determined from the interference patterns and can becompared to a specified value. Manufacturing conditions, e.g., apressure or vacuum applied to plates 452,453 can be changed to modifythe cell gap Δg if a difference between the specified value and thedetermined cell gap exceeds tolerances. This process can be repeateduntil achieving the desired cell gap. Liquid crystal medium is thenintroduced into the cell. The amount of liquid crystal medium to beadded can be determined from the measured spatial property of the cell.This can avoid over- or underfilling the cell. The filling process canalso be monitored by observing interference signals from the surfaces480,482. Once the cell has been filed, additional low coherenceinterference patterns are obtained to monitor the cell gap Δg (or otherspatial property). Again, the manufacturing conditions can be changed sothat the cell gap is maintained or brought within tolerances.

LASER SCRIBING AND CUTTING

Lasers can be used to scribe objects in preparation for separatingdifferent, concurrently manufactured structures, e.g., microelectronicsstructures. The quality of separation is related to the scribingconditions, e.g., laser focus size, laser power, translation rate of theobject, and scribe depth. Because the density of features of thestructure may be large, the scribe lines may be adjacent thin film orlayers of the structures. Interfaces associated with the thin film orlayers may create interference patterns that appear when interferometryis used to determine the scribe depth. The methods and systems describedherein can be used to determine the scribe depth even in the presence ofsuch adjacent films or layers.

An exemplary method can include scribing one or more electronicstructures and separating the structures along the scribe lines. Beforeand/or after separation, low coherence interference signals can be usedto determine the depth of scribe. Other scribing conditions are known,e.g., laser spot size, laser power, translation rate. The scribe depthcan be determined from the interference signals. The quality ofseparation as a function of the scribing conditions, including thescribe, depth, can be determined by evaluating the separated structures.Based on such determinations, the scribing conditions necessary toachieve a desired separation quality can be determined. During continuedmanufacturing, low coherence interference signals can be obtained fromscribed regions to monitor the process. Scribing conditions can bechanged to maintain or bring the scribe properties within tolerances.

Other aspects, features, and embodiments are within the scope of thefollowing claims.

1. An optical system, comprising: a photolithography system configuredto illuminate a portion of an object with a light pattern, thephotolithography system comprising a reference surface; a low coherenceinterferometer having a reference optical path and a measurement opticalpath, light that passes along the reference optical path reflecting atleast once from the reference surface and light that passes along themeasurement optical path reflecting at least once from the object; and adetector configured to detect a low coherence interference signalcomprising light that has passed along the reference optical path andlight that has passed along the measurement optical path, the lowcoherence interference signal being indicative of a spatial relationshipbetween the reference surface and the object.
 2. The optical system ofclaim 1, wherein the photolithography system includes an illuminationoptic having an illumination optic surface, light of the light patterntraveling along an optical path that includes the illumination opticsurface, wherein the illumination optic surface and the referencesurface are at least partially coextensive.
 3. The optical system ofclaim 1, wherein the light that passes along the measurement opticalpath reflects at least once from the portion of the object to beilluminated by the photolithography system.
 4. The optical system ofclaim 1, wherein the light of the low coherence interference signal thathas passed along the reference optical path and the light of the lowcoherence interference signal that has passed along the measurementoptical path have a range of optical path length differences, the rangebeing at least 20% of a coherence length of the low coherenceinterferometer.
 5. The optical system of claim 4, wherein the range isat least as great as the coherence length of the low coherenceinterferometer.
 6. The optical system of claim 1, wherein the detectorcomprises a plurality of detector elements each configured to detect arespective low coherence interference signal, each low coherenceinterference signal comprising light that has passed along a respectivedifferent portion of the reference optical path and light that haspassed along a respective different portion of the measurement opticalpath, each low coherence interference signal being indicative of aspatial relationship between a different point of the object and thereference surface.
 7. The optical system of claim 6, comprising: aprocessor configured to: determine the spatial relationship between eachof the different points of the object and the reference surface based onat least a respective one of the low coherence interference signals. 8.The optical system of claim 7, comprising a translation stage formanipulating a relative position and orientation between the object andthe photolithography system and wherein the processor is configured to:modify a relative position of the object and the photolithography systembased on the spatial relationships.
 9. The optical system of claim 6,wherein the light of each low coherence interference signal that haspassed along the respective different portion of the reference opticalpath and the light of the low coherence interference signal that haspassed along the respective different portion of the measurement opticalpath have a range of optical path length differences, the range being atleast 20% of a coherence length of the low coherence interferometer. 10.A method, comprising: positioning an object generally along an opticalpath of a photolithography system; reflecting a first portion of lightfrom a light source from a reference surface of the photolithographysystem; reflecting a second portion of light from the light source fromthe object; and forming a low coherence interference signal comprisinglight reflected from the reference surface and light reflected from theobject, the low coherence interference signal indicative of a spatialrelationship between the object and the photolithography system.
 11. Themethod of claim 10, comprising: reflecting a respective first portion oflight from the light source from each of a plurality of locations of thereference surface of the photolithography system; reflecting arespective second portion of light from the light source from each of aplurality of locations of the object; and forming plurality of lowcoherence interference signals, each comprising light reflected from arespective one of the different locations of the reference surface andlight reflected from a respective one of the different locations of theobject, each low coherence interference signal being indicative of aspatial relationship between at least one of the different locations ofthe object and the photolithography system.
 12. The method of claim 10,comprising performing the reflecting the first portion of light and thereflecting the second portion of light after positioning the object. 13.The method of claim 10, further comprising changing a relative positionof the object and the reference surface based on the spatialrelationship.
 14. The method of claim 10, wherein the reference surfaceis a surface of an optic of the photolithography system.
 15. The methodof claim 14, further comprising using the photolithography system toproject an ultraviolet light image onto the object, light forming theultraviolet image passing along an optical path including the surface ofthe optic.
 16. The method of claim 10, wherein the object includes asubstrate and a thin film having an outer surface and the formingcomprises combining light reflected from the reference surface and thelight reflected from the outer surface of the thin film, and the spatialrelationship is between the outer surface of the thin film and thephotolithography system.
 17. The method of claim 16, wherein the lightof the second portion of light from the light is substantiallyattenuated by the thin film.
 18. The method of claim 16, wherein thethin film includes photoresist and the light of the second portion oflight from the light source has an energy insufficient to expose thephotoresist.
 19. The method of claim 10, wherein the object includes asubstrate and a thin film having an outer surface and the formingcomprises combining light reflected from the reference surface and lightreflected from the substrate, and the spatial relationship is betweenthe substrate and the photolithography system.
 20. The method of claim19, wherein the reflecting a second portion of light from the lightsource from the object comprises irradiating the object at Brewster'sangle.
 21. The method of claim 10, wherein the forming comprises usingan interferometer and the light of the low coherence interference signalthat has passed along the reference optical path and the light of thelow coherence interference signal that has passed along the measurementoptical path have a range of optical path differences, the range beingat least 20% of a coherence length of the interferometer.
 22. The methodof claim 21, wherein the reflecting a second portion of light from theobject comprises directing irradiating the object at an angle ofincidence of at least 50°.
 23. A system for determining a spatialproperty of an object, comprising: a light source; an optical systemconfigured to: illuminate the object at a grazing angle of incidencewith a first portion of light from the light source, at least some ofthe first portion of light reflecting from the object; illuminate aprocessing tool with a second portion of light from the light source, atlease some of the second portion of light reflecting from a surface ofthe processing tool; vary the optical path difference between the lightreflected from the object and light reflected from the surface of theprocessing tool, combined, over a range of optical path differences,light reflected from the object and light reflected from the surface ofthe processing tool; and a detector configured to detect the lightcombined over the range of optical path differences as a plurality ofinterference fringes each having a peak amplitude, the range of opticalpath differences being sufficient to modulate the peak amplitudes of theinterference fringes.
 24. The system of claim 23, wherein the range ofoptical path differences is at least as great as a coherence length ofthe optical system.
 25. The system of claim 23, comprising a processor,the processor is configured to: determine a spatial property of theobject based on the plurality of interference fringes.
 26. The system ofclaim 23, wherein: the optical system is configured to: illuminate eachof a plurality of points of the object at a grazing angle of incidencewith a respective first portion of light from the light source, at leastsome of each respective first portion of light reflecting from theobject as a respective portion of reflected light; combine each portionof reflected light with a corresponding second portion of light derivedfrom the same light source to prepare respective combined light; and thedetector comprises: a plurality of detector elements, each configured todetect a respective plurality of interference fringes, each respectiveplurality of interference fringes comprising contributions from arespective combined light, the combined light of the each plurality ofinterference fringes having a range of optical path length differences,each range of optical path length differences being sufficient tomodulate the peak amplitudes of the corresponding interference fringes.27. The system of claim 26, comprising a processor, the processor isconfigured to: determine a spatial property of each of the points basedon the respective plurality of interference fringes.
 28. The system ofclaim 25, wherein the object comprises a substrate and an overlying thinfilm, the thin film having an outer surface, and wherein the spatialproperty is a spatial property of the outer surface of the thin film.29. The system of 28, wherein the processor is in communication with atranslation stage configured to change a relative position between theobject and a reference, and wherein the processor is configured tochange the relative position based on the spatial property.
 30. Amethod, comprising: illuminating an object a grazing angle of incidencewith a first portion of light from a light source, at least some of theilluminating light reflecting from the object; illuminating a surface ofa processing tool with light from a light source, at least some of theilluminating light reflecting from the processing tool; providing arange of optical path differences between the light reflected from theobject and the portion of light derived from the light source;combining, over the range of optical path differences, light reflectedfrom the object and light reflected from the surface of the processingtool; detecting the light combined over a range of optical pathdifferences as a plurality of interference fringes each have a peakamplitude, the range of optical path differences being sufficient tomodulate the peak amplitudes of the interference fringes.
 31. The methodof claim 30, wherein the combining comprises using an interferometerhaving a coherence length, the range of optical path differences beingat least as great as the coherence length.
 32. The method of claim 31,wherein the detecting comprises detecting at least a portion of aninterference pattern comprising the interference fringes and the methodcomprises determining a spatial property of the object based on the atleast a portion of an interference pattern, and outputting informationrelated to the spatial property.
 33. The method of claim 32, wherein theobject comprises a substrate comprising an overlying layer ofphotoresist having an outer surface and the spatial property is aspatial property of the outer surface, and the processing tool comprisesa photolithography system.
 34. The method of claim 33, comprisingchanging a relative position between the photolithography system and theobject based on the spatial property of the outer surface.
 35. Themethod of claim 32, wherein the object comprises a portion of a liquidcrystal display.
 36. The method of claim 32, further comprising scribingthe object and wherein the processing tool comprises a scribing systemand the spatial property is a spatial property of a first scribed lineformed by the scribing.
 37. The method of claim 36, comprising furtherscribing the object or another object and changing a parameter of thefurther scribing based on the spatial property of the first scribedline.
 38. The method of claim 32, wherein the processing tool comprisesa solder bump manufacturing tool and the object comprises a structureduring solder bump manufacturing.
 39. The method of claim 38, whereinthe spatial property is a spatial property of a portion of the objectnon-wettable by solder.
 40. An apparatus, comprising: a photolithographysystem configured to illuminate a portion of an object with a lightpattern, the photolithography system comprising a reference surface; aplurality of detector elements; an optical system operable as aninterferometer and as a triangulation system, the interferometer havinga reference optical path and a measurement optical path, light thatpasses along the reference optical path reflecting at least once fromthe reference surface and light that passes along the measurementoptical path reflecting at least once from the object, wherein theplurality of detector elements are configured to detect an interferencesignal comprising light that has passed along the reference optical pathand light that has passed along the measurement optical path, theinterference signal being indicative of a spatial relationship betweenthe reference surface and the object; and the triangulation system isconfigured to determine a spatial relationship between the object andthe reference surface based on a spatial relationship between an imageof the object and an image of the reference surface.
 41. An apparatus,comprising: a photolithography system configured to illuminate a portionof an object with a light pattern, the photolithography systemcomprising a reference surface; a plurality of detector elements; anoptical system configured to illuminate a portion of the object withfirst light, illuminate the reference surface with second light, andform an image of the first light and an image of the second light on thedetector; and a processor configured to determine a spatial relationshipbetween the object and the reference surface based on a spatialrelationship between the image of the first light and the image of thesecond light.
 42. The apparatus of claim 41, wherein the processor isconfigured to determine the spatial relationship between the image ofthe first light and the image of the second light as a function of thedetector elements.
 43. The apparatus of claim 41, comprising apositioner configured to modify a relative position between the objectand the reference surface, wherein the processor is configured tooperate the positioner based on the spatial relationship between theobject and the reference surface.
 44. The apparatus of claim 41, whereinthe portion of the object illuminated with light is a portion of theobject to be illuminated by the photolithography system.
 45. Theapparatus of claim 44, wherein the photolithography system includes anoptic having a surface at least partially coextensive with the referencesurface.
 46. The apparatus of claim 45, wherein the optical systemcomprises: an interferometer having a reference optical path and ameasurement optical path, light that passes along the reference opticalpath reflecting at least once from the reference surface and light thatpasses along the measurement optical path reflecting at least once fromthe object; and wherein the plurality of detector elements areconfigured to detect an interference signal comprising light that haspassed along the reference optical path and light that has passed alongthe measurement optical path, the interference signal being indicativeof a spatial relationship between the reference surface and the object.